Systems for the transmission of the digital information, over line wire links, microwave or other radio links, are well known to the art. Some of these systems, either now operating or planned for operation, are thousands of miles in length or more. For those systems which employ synchronous data handling a clock must be available at each data handling locality which is synchronized with the clock at the originating station. To enable any station in the system to communicate with any other station, then, each station must have a local clock available which is synchronized with every other local clock at every other station. In order to effect this a single station is selected as the master and its clock employed for data transmission. The adjacent station generates a synchronous clock for distribution and retransmission. Of course, due to signal degradation, as a result of transmission, the remotely transmitted signal (or clock) must be reconditioned or regenerated at each of the stations. In order to provide a stable synchronized signal for local distribution a phase lock loop, which is a well known circuit configuration, is an obvious choice. However, the very high data transmission rates, up to 20 M bits per second, coupled with the serious consequences at any station of losing the clock, require that in addition to stability, the local clock generation equipment must be highly reliable.
One well known method of increasing reliability of any apparatus is to provide that apparatus in redundant form, that is to provide more than one copy of the apparatus and, when a failure of an operating apparatus is indicated a switch over can be effected to the redundant apparatus. It should be apparent to those skilled in the art that due to the high data rates employed in typical data transmission systems, one cannot rely upon manual switch over between redundant apparatus. Therefor, automatic operating apparatus must be provided to effect a switching operation. In addition, as a further corollary of the reliability requirements, the automatic switching apparatus must be capable of sensing of any one of a number of possible failures (including failure of the failure detecting apparatus) and properly responding thereto in a short a time as posssible.
Merely providing redundant apparatus in automatic equipment to switch out a "failed" unit and switch in a "good" unit will not, however, porvide a complete solution. In addition to the possibility that one of the redundant apparatus may fail, this system must also handle the possibility that the local station will not receive the remotely transmitted signal, or if a remotely transmitted signal is received it is so degraded that synchronizing the local oscillator thereto will actually be detrimental to system operation. Thus, not only must the apparatus respond to effective loss of the remotely transmitted signal but some means must be provided for independently generating the signal and furthermore, such independent local generated signal must meet system specifications.